Polypropylene compositions have gained wide commercial acceptance and usage in numerous applications because of the relatively low cost of the polymers and the desirable properties they exhibit. Commercially available isotactic polypropylenes are polymers that have a highly linear structure, have relatively low melt strength, and exhibit poor strain hardening behavior in the molten state. While these isotactic polypropylenes are relatively easy to produce, they have very limited applications in processes such as thermoforming, foaming, blow molding, film molding, extrusion coating, because of their poor extensional performance, poor film toughness properties, and low melt strength.
Polymers containing long-chain branches, on the other hand, have great value in processing techniques that demand high melt strength.
However, there are substantial difficulties in creating long-chain branched polyolefin, particularly polypropylene. Known routes to produce polypropylene in commercial scale, such as Ziegler-Natta and Metallocene catalysis, usually produce highly linear and highly stereospecific polymers. Polymers with a branched or long-chain branched structure have been reported using Metallocene catalysts, although there are significant limitations in the polymerization process and catalyst performance that impose a challenge for production in commercial scale.
In another example, very small amounts of long-chain branches are known to be produced and controlled during the polymerization of high density polyethylene (HDPE) using chromium catalyst. The amount of branches or long-chain branches, along with molecular weight (MW) and molecular weight distribution (MWD) are factors to determine the melt elasticity of the polyethylene (PE), which largely defines its commercial processing characteristics.
There are also processes to introduce long-chain branches into polyolefins via post polymerization. For instance, a long-chain branched polypropylene can be prepared through a coupling reaction of polypropylene and sulfonyl azides. However, there are disadvantages using sulfonyl azide chemistries. For example, some sulfonyl azides can be highly reactive, making reaction control difficult due to the relative lower temperatures (below 140° C.) in which the nitrene radical is formed, which can consequently lead to an uneven distribution of linkages in the polypropylene sample. Furthermore, highly reactive sulfonyl azides compounds may increase the risk for explosion and the generation of toxic by-products.
There thus remains a need in the art to develop an improved process to prepare polyolefins having long-chain branches that can provide high melt strength.